hmmm - if this was true, my bike would be all fried up by now. I am forced to conclude that there is some error in the above statement.
Can you jump start the bike through the battery tender connector?
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That is a good point dmfdmf. You zeroed right in on the weak spot. Shunt power has the potential to be a problem though might not actually be an issue.
Referring to the SV650 charging diagram above, the electrical parts that would be stressed would be the SCRs and the Stator coils. The rectifying diodes and the Control IC would not be affected.
Suppose the running car puts out, say, 14.5v, but the SV650 regulator SCRs are programmed to shunt at 14.0v. Seeing 14.5v would trigger the SCRs to continuously short the stator during the entire time the car is connected.
Damage to the SCRs and stator would depend on 1) Shunt duration, 2) How much power the stator is producing while shunted 3) How much margin is in the SV650 R/R SCR and stator design.
-Presumably, the duration would be short. Connect the jumper cables, start the bike, remove the jumpers.
-Stator power output would be low at cranking and idle rpm. The only time the stator would generate significant power is if the SV engine were held revved.
-How much design margin is in the SV650 R/R and stator might be inferred by SV650 race bikes. Race bikes run without lights, minimal electrical load, so the SCRs would be shunting a good amount of time. Additionally, the rpm of race bikes often pushes redline, so stator power output would be near max. Some of these SV bikes do 24hr endurance racing without blowing up the stator or R/R, so it seems there is a fair amount of margin for long-duration shunting at high Stator power.
Conclusion, the SCRs and Stator are not likely to be damaged.
As for other motorcycle electrics being damaged, given the motorcycle and car have the same voltage specs, why would the motorcycle be damaged and not the car?
Just because a car battery has massive current capacity relative to a motorcycle is immaterial. As Ziff said, given the same 12v charging systems, the bike will only draw the current needed. A 20A wall outlet can power a 10A hairdryer but will not blow up a mA nightlight. For a given voltage, the load defines the current. Ohm's law.
This topic is a fun intellectual discussion but basically moot. Even if jump-starting a motorcycle from a running car has zero chance of causing damage (likely true, no damage), a car battery has sufficient power to crank a motorcycle starter without the car running. No need to start the car.
@TeeRiver
Now, that was awesome. May I ask what you do for a living?
EDIT: Looked this up a bit. Seems like the shunting method is a somewhat dated method of limiting the output voltage of the generator to a level that will not fry the vehicle electronics or overcharge the battery. Excess energy is simply dumped to ground as the shunting diodes close over a certain voltage. A couple of things:
Now, that was awesome. May I ask what you do for a living?
This part I don't quite get. I am assuming that the shunt mechanism is a protection mechanism. I can only guess that it is in place to detect that the battery is already at max voltage (fully charged), so it stops overcharging it. But if it is a protection mechanism, then it should not result in a catastrophic short (as suggested by @dmfdmf).
EDIT: Looked this up a bit. Seems like the shunting method is a somewhat dated method of limiting the output voltage of the generator to a level that will not fry the vehicle electronics or overcharge the battery. Excess energy is simply dumped to ground as the shunting diodes close over a certain voltage. A couple of things:
- The shunting happens as a matter of course ALL the time - as the generator routinely creates excess voltage (depending on the RPM).
- Since the car and MC voltages are very similar, no excessive shunting will happen because of the jump start.
- I have to now agree with @TeeRiver that it was pretty cool of @dmfdmf to think about this issue in the context of a jump start.
Oh, good. Finally, a definitive confirmation. We should sticky this on the internet
Noted. Thanks!
When my R/R went is only had to hit 16v for a short period for it to ruin the Battery. MC Batteries are not robust.
The MC battery I'm using came with specs/warning to only us a maximum charging rate of 10% of the Amp rating of the battery and in this case 0.9 amps ( 900 mA).
HTH
1. Right, shunting is by design however it only happens when the stator puts out enough power to exceed regulation voltage (typ 14.5v). For example, at idle, battery voltage may drop to 14v or lower. Whenever the regulator sees less than 14.5v, the SCRs will be turned off, zero shunting.
2. No. Excess shunting (continuous, which is the potential problem) will happen if the car's charging system puts out, say, 15v when the motorcycle regulator is set to 14.5v. If that happens, the MC regulator will try to regulate down to 14.5v by shunting 100% continuous, which might generate enough heat to do damage. If the car is not running, then the car voltage will be 12.8v max, not enough to tell the MC regulator to shunt.
3. Yes, that was keen insight on dmfdmf's part.
I think I read something about that. A situation where a running car's alternator puts out large current to the vehicle being jumped. When the cables are suddenly removed, the regulator is unable to react fast enough (mechanical regulator, or the alternator's magnetic field does not collapse fast enough), resulting in a current surge which causes a voltage spike. But in this situation, the spike would happen to the donor car, not the receiving motorcycle/vehicle.
One reason you would want to have the donor car running is to generate high enough voltage to actually charge the receiving vehicle's battery. If the donor car is not running, max battery voltage is only 12.8v, barely enough to provide a charge. Years ago, growing up in NJ, cold winters and ancient battery technology had me jumpstarting my car or giving a jump to someone in need, all the time. Sometimes a successful jump required connecting the cars together for 15min with the donor car's engine at 2k rpm to generate enough charging current. Ah, the good ole days!
My hunch on how the story got started is 1) What you said, 2) A vehicle that needs a jump probably has a problem in the first place. After doing the jump, yup, the problem is still there.
3) It is easy to accidentally connect cables reverse-polarity and do all kinds of damage.
Right, that is the mechanism but probably not a problem in real life.
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